Exposure to environmental mutagens contributes to an increased risk of somatic mutation, and consequently, an increased risk of birth defects and of cancer. To help better assess the mutagenic behavior of environmental agents, the objective of this proposal is to produce mice that can be used as detectors of environmental mutagens. The characteristics of the detector mice will not only identify mutagenic agents but will also define the target organs for each mutagen, and will establish the existence (or lack thereof) of a dose-response relationship. Since this assay utilizes whole animals, it will be capable of distinguishing tissue-specific promutagen metabolic activation. If desired, the nature of the detected mutations can be directly determined at the nucleotide level. The mutagenesis target will be a bacterial lacI transgene which encodes the lac repressor. The indicator gene will be a bacterial lacZ transgene which encodes bacterial beta-galactosidase. The transgenic detector mice will carry a single lacI transgene and one or two lacZ transgenes. If the lacI transgene is functional the lacZ transgenes will be repressed, and cells of detector mice will not stain for beta-galactosidase activity. If a cell incurs a mutation that inactivates the lacI gene, that cell and its progeny will stain for beta-galactosidase against an unstained background. Thus, by sectioning and staining sections of individual organs and tissues, following administration of a substance, one can identify agents that are mutagenic, the target organs most affected by that mutagen, the effect of route of administration upon the distribution of target organs, and the relative potency of the agent. The nature of the mutation can be determined by extracting DNA from the affected cells, amplifying the mutant lacI gene by the polymerase chain reaction (PCR), and sequencing the amplified product. Lastly, a relationship, or absence thereof, can be established between organs and tissues that incur mutations and those that later produce tumors.